From ae498864bb638e597b589c52ff27237835625281 Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Wed, 10 Apr 2024 21:36:39 +0200
Subject: ld.so: load libs with same name experiments
---
src/development/ld.so.md | 5 +++++
src/development/ldso/samename/Makefile | 13 ++++++++++++
src/development/ldso/samename/lib.c | 9 +++++++++
src/development/ldso/samename/main.c | 36 ++++++++++++++++++++++++++++++++++
4 files changed, 63 insertions(+)
create mode 100644 src/development/ldso/samename/Makefile
create mode 100644 src/development/ldso/samename/lib.c
create mode 100644 src/development/ldso/samename/main.c
(limited to 'src/development')
diff --git a/src/development/ld.so.md b/src/development/ld.so.md
index 9b03c45..fd271bc 100644
--- a/src/development/ld.so.md
+++ b/src/development/ld.so.md
@@ -199,6 +199,10 @@ symbol=default_main; lookup in file=/lib64/ld-linux-x86-64.so.2 [0]
./main: error: symbol lookup error: undefined symbol: default_main (fatal)
```
+## Load lib with same name from different locations
+The sources in [ldso/samename][src-samename] show some experiments, loading the
+libs with the same name but potentially from different locations (paths).
+
## Dynamic Linking (x86_64)
Dynamic linking basically works via one indirect jump. It uses a combination of
function trampolines (`.plt` section) and a function pointer table (`.got.plt`
@@ -260,3 +264,4 @@ Using `radare2` we can analyze this in more detail:
As we can see the offset from relocation at index `0` points to `GOT[3]`.
[src-deepbind]: https://github.com/johannst/notes/tree/master/src/development/ldso/deepbind
+[src-samename]: https://github.com/johannst/notes/tree/master/src/development/ldso/samename
diff --git a/src/development/ldso/samename/Makefile b/src/development/ldso/samename/Makefile
new file mode 100644
index 0000000..f35a1e3
--- /dev/null
+++ b/src/development/ldso/samename/Makefile
@@ -0,0 +1,13 @@
+all:
+ mkdir -p foo bar
+ gcc -g -o foo/lib.so lib.c -shared -fPIC -DNAME=\"foo\"
+ gcc -g -o bar/lib.so lib.c -shared -fPIC -DNAME=\"bar\"
+ gcc -g -o main main.c -ldl
+ ./main foo/lib.so bar/lib.so
+ LD_LIBRARY_PATH=foo:bar ./main lib.so lib.so
+ LD_LIBRARY_PATH=bar:foo ./main lib.so lib.so
+ LD_LIBRARY_PATH=foo ./main lib.so foo/lib.so
+ LD_LIBRARY_PATH=foo ./main lib.so bar/lib.so
+
+clean:
+ $(RM) -r foo bar main
diff --git a/src/development/ldso/samename/lib.c b/src/development/ldso/samename/lib.c
new file mode 100644
index 0000000..d467d7f
--- /dev/null
+++ b/src/development/ldso/samename/lib.c
@@ -0,0 +1,9 @@
+#include <stdio.h>
+
+#ifndef NAME
+#define NAME __FILE_NAME__
+#endif
+
+void moose() {
+ puts(NAME " says moose");
+}
diff --git a/src/development/ldso/samename/main.c b/src/development/ldso/samename/main.c
new file mode 100644
index 0000000..6d6a8ea
--- /dev/null
+++ b/src/development/ldso/samename/main.c
@@ -0,0 +1,36 @@
+#include <assert.h>
+#include <dlfcn.h>
+#include <stdio.h>
+#include <string.h>
+
+void print_libso() {
+ FILE* m = fopen("/proc/self/maps", "r");
+ assert(m);
+
+ char line[256];
+ while (fgets(line, sizeof(line), m)) {
+ if (strstr(line, "lib.so")) {
+ printf("%s", line);
+ }
+ }
+ fclose(m);
+}
+
+int main(int argc, char* argv[]) {
+ for (int i = 1; i < argc; ++i) {
+ void* h = dlopen(argv[i], RTLD_LAZY | RTLD_GLOBAL);
+ if (!h) {
+ puts(dlerror());
+ return 1;
+ }
+
+ void (*next)() = dlsym(h, "moose");
+ assert(next);
+ next();
+
+ // leak lib, we want to priint the mmap.
+ }
+
+ print_libso();
+ return 0;
+}
--
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